This proposal represents a competitive renewal for a close collaborative effort between two research groups, one with expertise in HIV-1 biology (Chen) and the other with expertise in structural studies of macromolecules (Feigon). HHR23A and its homolog HHR23B were first identified as components of the DNA damage repair nucleotide excision repair (NER) complex. Their function is as yet unclear but within the last two years they have been shown to bind a number of proteins involved in DNA repair and proteasome degradation through at least 3 distinct domains. Considerable new insight has emerged from our studies and other groups on a central role for HHR23A/B in linking cellular processes of DNA repair and ubiquitin/proteasome degradation. HIV-1 Vpr is a protein that has multiple functions, including the ability to arrest cells in the G2/M phase of the cell cycle and to induce apoptosis. The mechanism of the cell cycle arrest and apoptosis remains unclear. We proposed the hypothesis in 1995 that cell cycle arrest was initiated through cellular signaling pathways involving response to DNA damage and apoptosis a consequence of prolonged cell cycle arrest. In support of that hypothesis, we identified HHR23A as a Vpr-binding protein. Failure to repair DNA initiates cell cycle arrest and the ubiquitin-dependent degradation of cell cycle inhibitors may be critical for cell cycle progression. Thus, we believe that Vpr/HHR23A/B interaction is a key step in Vpr mediated cell cycle arrest. The overall objective of the proposed studies is to develop a comprehensive understanding of the structure/function relationships of HHR23A/B in cellular function, particularly as it relates to DNA repair, cell cycle, and ubiquitin/proteasome degradation pathways. This understanding will, in turn, provide us with critical information needed to understand the role of Vpr/HHR23A interaction and to develop potential AIDS therapeutic agents based upon the interaction between these proteins.